This invention relates generally to improvements in the laminated boards used in electronic circuitry.
Printed wiring boards (PWB's) are composites of an insulating core and one or more planar layers of circuitry. The insulating core comprises a thermosetting polymer, such as an epoxy resin, and a suitable reinforcing material, such as glass cloth. The core laminate may be faced with a layer of copper foil to provide an electrically conductive path. The circuit patterns are created by photoimaging and etching of unwanted copper. Where there are multiple layers,, the circuits are interconnected by through-plated holes precisely positioned in predetermined locations. A major problem with multi-layer composites is misregistration of the layers. When manufacturing the PWB's, stresses are developed and released. The layers may expand or contract relative to each other and create what are called registration problems. The layers may not be interconnected at the intended places and possibly may cause short circuits. Failure to interconnect layers can be a cause for either failure or rejection of the composite.
The present inventors have investigated the reasons for registration problems and found that the difference in coefficient of thermal expansion between the reinforced laminate and the copper foil creates much of the problem and further that the laminates used commonly in the industry do not respond uniformly when heated. Consequently, the inventors have found methods of providing uniform thermal expansion and matching of the thermal expansion of the laminate and the copper layer, as discussed in detail below.